Single-crystal Sapphire Substrates Research Articles

Anderson localization enhanced ferromagnetism in Zn0.95Co0.05O

We report an enhancement in the ferromagnetic characteristics of Zn0.95Co0.05O thin films due to the localization of charge carriers. Epitaxial thin films of Zn0.95−xCo0.05GaxO (x=0–0.05) were grown on single-crystal sapphire (0001) substrates by pulsed laser deposition technique. The role of charge carrier localization on the electrical and magnetic properties of ZnO:Co was studied by introducing Ga into the system. It was observed that Ga plays a significant role in affecting both the electrical transport mechanism as well as the magnetization of the material. Electrical resistivity of Zn0.95Co0.05O at room temperature was ∼96 mΩ cm and exhibited metal-like temperature dependence, although strongly influenced by electron-electron (e-e) interactions. Strong e-e interaction was understood to arise because of the randomness introduced in the crystal potential of ZnO by the cobalt dopants. As the Ga dopants are introduced, randomness in crystal potential and hence the disorder further increases resulting in the Anderson localization of the carriers. The increase in localization was accompanied by a significant enhancement in the magnetic moment from 0.75μB/Co in Zn0.95Co0.05O films to 1.6μB/Co in Zn0.90Co0.05Ga0.05O.

Tailoring of inclined crystal orientation in layered cobaltite thin films for the development of off-diagonal thermoelectric effect

A highly inclined CoO2 plane orientation was realized in CaxCoO2 thin films grown on sapphire single crystal substrates. The inclination angle of the CoO2 plane orientation was controlled by changing the inclination angle of the sapphire(0001) plane with respect to the substrate surface and additionally by introducing an epitaxial buffer layer of hematite. We demonstrate that the highly inclined crystal orientation results in a unique off-diagonal thermoelectric effect with a potential to produce a voltage signal of up to 600 mV along the film in-plane direction by a unit temperature difference along the film out-of-plane direction.

The metal-insulator transition in vanadium dioxide: A view at bulk and surface contributions for thin films and the effect of annealing

Vanadium dioxide is investigated as potential oxide barrier in spin switches, and in order to incorporate VO2 layers in complex multilayer devices, it is necessary to understand the relation between bulk and surface/interface properties. Highly oriented VO2 thin films were grown on (0001) sapphire single crystal substrates with reactive bias target ion beam deposition. In the analysis of the VO2 films, bulk-sensitive methods [x-ray diffraction (XRD) and transport measurements] and surface sensitive techniques [photoelectron spectroscopy (PES) and scanning tunneling microscopy and spectroscopy] were employed. The samples were subjected to heating cycles with annealing temperatures of up to 425 and 525K. Prior to annealing the VO2 films exhibit the transition from the monoclinic to the tetragonal phase with the concurrent change in conductivity by more than a factor of 103 and their phase purity is confirmed by XRD. Annealing to 425K and thus cycling across the metal-insulator transition (MIT) temperature has no impact on the bulk properties of the VO2 film but the surface undergoes irreversible electronic changes. The observation of the valence band with PES during the annealing illustrates that the surface adopts a partially metallic character, which is retained after cooling. Annealing to a higher temperature (525K) triggers a modification of the bulk, which is evidenced by a considerable reduction in the MIT characteristics, and a degradation in crystallite morphology. The local measurement of the conductivity with scanning tunneling spectroscopy shows the transition of the surface from predominantly semiconducting surface prior to annealing to a surface with an overwhelming contribution from metallic sections afterward. The spatial distribution of metallic regions cannot be linked in a unique manner to the crystallite size or location within the crystallites. The onset of oxygen depletion at the surface is held responsible for this behavior. The onset of bulk modification at higher temperatures is most likely linked to oxygen loss and effusion along the grain boundaries and concurrent onset of sintering. Our study focuses on the comparison of the MIT in the bulk and at the surface of thin VO2 layers and establishes an irreversible modification of the crystallite structure and surface for temperatures exceeding the MIT. The surface modification impacts on the strategies which will be employed to build the metallic contacts to VO2 layers.

Blue Luminescent Center in Undoped ZnO Thin Films Grown by Plasma-assisted Molecular Beam Epitaxy

ZnO thin film was grown on a sapphire single crystal substrate by plasma assisted molecular beam epitaxy. In addition to near band edge (NBE) emissions, both blue and green luminescences are also observed together. The PL intensity of the blue luminescence (BL) range from 2.7 to 2.9 eV increased as the amount of activated oxygen increased, but green luminescence (GL) was weakly observed at about 2.4 eV without much change in intensity. This result is quite unlike previous studies in which BL and GL were regarded as the transition between shallow donor levels such as oxygen vacancy and interstitial zinc. Based on the transition level and formation energy of the ZnO intrinsic defects predicted through the first principle calculation, which employs density functional approximation (DFA) revised by local density approximation (LDA) and the LDA+U approach, the green and blue luminescence are nearly coincident with the transition from the conduction band to zinc vacancies of <TEX>$V^{2-}_{Zn}$</TEX> and <TEX>$V^-_{Zn}$</TEX>, respectively.

Optical and structural characteristics of Ga-doped ZnO films

The n-ZnO films doped with Ga to the content 2.5 at % are produced by pulse laser deposition onto the (0001) oriented single crystal sapphire substrates. The transmittance spectra of the ZnO films in the range from 200 to 3200 nm are studied in relation to the Ga dopant content. It is established that an increase in the Ga content shifts the fundamental absorption edge to the blue region, but reduces the transparency of the ZnO films in the infrared spectral region. The dependence of the band gap on the level of doping with Ga is determined. The photoluminescence spectra of the ZnO films doped to different levels are recorded. It is established that the PL intensity and peak position vary unsteadily with the level of doping. X-ray diffraction studies of the structure of the films are carried out. It is found that the crystallographic parameters (the lattice constant c) of the ZnO film depend on the Ga dopant content and the conditions of deposition of the films.

Structural Characterization of PLD-grown Nanometer Size Ferromagnetic NiFeMo Films

AbstractThin films of 3 different thicknesses each of Ni83.2Fe3.3Mo13.5 and Ni83.1Fe6.0Mo10.9alloys have been grown using Pulsed Laser Deposition (PLD) technique. Our motivation is to investigate the magnetic properties of a few nm thick Ni alloys with mostly Mo (4d element) addition since the corresponding soft ferromagnetic bulk alloys have shown very small coercivity of ˜ 0.1 Oe. Detailed structural characterization has been undertaken before probing the magnetic properties. Arc melted alloy buttons after homogenization are used directly as targets for the deposition. Films were deposited on single crystal Sapphire (0001) substrates using excimer laser. The structural characterization has been done by X-ray diffraction (XRD), X-ray reflectivity (XRR), Energy dispersive x-ray spectroscopy (EDS), and Atomic force microscopy (AFM). The X-ray diffraction pattern shows that the films are highly textured and grown along [111] direction of the alloys. They have high lattice strain which makes the films highly resistive and the resistance decreases with increasing thickness. The EDS measurements, using Scanning electron microscope (SEM), indicate that the compositions of the films are almost the same as those of the targets. Thickness, roughness, and density gradients are estimated using XRR measurements. The thinner films have higher roughness compared to the thicker ones for both the compositions. The films have density gradient across their thickness. The bottommost low density layer has high roughness which is supposed to be the result of initial non uniform coverage of the substrate. The density of the middle layer, having the lowest roughness, is approximately near the bulk value and it increases with increasing film thickness. The change in density is not due to the variation of composition; instead it is due to the variation of void densities in the layers. The topmost layer, having the lowest density and the highest roughness, is interpreted as a porous layer which is also evident from the AFM images.

Controlled synthesis of hydroxyapatite-based coatings for biomedical application

Here we report the controlled synthesis of hydroxyapatite (HAP) HAP/Ti–alloy composite thin films using pulsed laser deposition (PLD) technique. A KrF excimer laser (wavelength 248 nm, 25 ns pulse duration, 2–3 J/cm 2) was used to grow thin films at a base vacuum of ∼ 10 − 6 mbar, and substrate temperatures ranging from room temperature to 500 °C. Films were deposited on Si (100) and Sapphire (0001) single crystal substrates. Selected films were annealed in air at 400 °C, or nitrogen/water vapor at 550 °C to study crystallization properties. Fourier transform infrared ipectroscopy (FTIR), x-ray diffraction (XRD), energy dispersive x-ray spectroscopy (EDS), and atomic force microscopy (AFM) characterization techniques were employed in the study of the films after deposition and after annealing. All as-deposited thin films were amorphous as seen by XRD and FTIR. AFM showed all films contained micron sized particles embedded into them. Density distribution and size of embedded particles was found to be much less for HAP/Ti–alloy composite thin films compared to pure HAP Films. In between the embedded particles the films were quite smooth. Root mean squared (RMS) roughness of pure HAP films was ∼ 100 nm and decreased to ∼ 50 nm as Ti–alloy content increased. Film roughness in between particles remained smooth to within 1 nm. Annealing of thin films on Si (100) at 400 °C in air resulted in very limited crystallization. However, annealing of thin films on sapphire at 550 °C in nitrogen/water vapor resulted in significant crystallization as seen in both XRD and FTIR with the added features of probable texturing resulting in the prominent presence of (002), and (112) diffraction peaks.

Growth of Nd:Gd3Ga5O12 Thin Films by Pulsed Laser Deposition for Planar Waveguide Laser

Pulsed laser radiation of a KrF excimer laser was used for the deposition of thin Nd 3+ doped Gd3Ga5O12 (Nd:GGG) films on yttrium aluminium garnet (YAG) and sapphire single crystal substrates. By variation of PLD-parameters such as temperature and processing gas pressure, amorphous and single crystalline thin films were produced. The morphology and the composition of the grown films were investigated by optical microscopy, scanning electron microscopy and electron dispersive X-ray spectroscopy. Thickness and structural properties of the deposited films were determined by optical reflection spectroscopy and X-ray diffraction, respectively. The optical properties of grown films on different substrates were compared. An amorphous Nd:Gd3Ga5O12 thin film on yttrium aluminium garnet was used for demonstration of an infrared waveguide laser. A planar wave guiding structure was formed in deposited film between two parallel grooves micromachined using laser radiation delivered by a femtosecond CPA-Laser-System. The resulting waveguides were polished and provided with resonator mirrors. With a 5% output coupler at 1064 nm, a laser threshold of 1080 mW and 0.2% slope efficiency were obtained.

Ferromagnets based on diamond-like semiconductors GaSb, InSb, Ge, and Si supersaturated with manganese or iron impurities during laser-plasma deposition

Properties of thin (30–100 nm) layers of diluted magnetic semiconductors based on diamond-like compounds III–V (InSb and GaSb) and elemental semiconductors Ge and Si doped with 3d impurities of manganese and iron up to 15% were measured and discussed. The layers were grown by laser-plasma deposition onto heated single-crystal gallium arsenide or sapphire substrates. The ferromagnetism of layers with the Curie temperature up to 500 K appeared in observations of the ferromagnetic resonance, anomalous Hall effect, and magneto-optic Kerr effect. The carrier mobility of diluted magnetic semiconductors is a hundred times larger than that of the previously known highest temperature magnetic semiconductors, i.e., copper and chromium chalcogenides. The difference between changes in the magnetization with temperature in diluted semiconductors based on III–V, Ge, and Si was discussed. A complex structure of the ferromagnetic resonance spectrum in Si:Mn/GaAs was observed. The results of magnetic-force microscopy showed a weak correlation between the surface relief and magnetic inhomogeneity, which suggests that the ferromagnetism is caused by the 3d-impurity solid solution, rather than ferromagnetic phase inclusions.

Critical Depth of Cut and Specific Cutting Energy of a Microscribing Process for Hard and Brittle Materials

Abstract This paper investigated the scribing process characteristics of the hard and brittle materials including single crystal silicon, STV glass, and sapphire substrate. Under various cutting angles, major process characteristics are examined including the groove geometry, specific cutting energy, and critical depth of cut at the onset of ductile-to-brittle cutting transition. As the cutting depth increases, groove geometry clearly reveals the ductile-to-brittle transition from the plastic deformation to a brittle fracture state. The material size effect in the ductile region as well as the transition in scribing behavior is well reflected by change in the specific cutting energy. Further, it is shown that the change of specific cutting energy as a function of the cutting depth can serve as a criterion for estimating the critical depth of cut. Such estimated critical depth of cut is confirmed by measurement from a 3D confocal microscope. The critical depths of cut for these hard materials are found to be between 0.1μm and 0.5μm depending on the materials and cutting angles.

Structural, electrical, and optical characterizations of epitaxial Zn1−xGaxO films grown on sapphire (0001) substrate

In this paper we report the structural, electrical, and optical properties of epitaxial Zn1−xGaxO films (x=0–0.05) grown on single crystal sapphire (0001) substrate by pulsed laser deposition technique. Structural and elemental analysis was performed using high-resolution x-ray diffraction (θ-2θ and Φ scan) and energy dispersive x-ray spectroscopy. Temperature dependent electrical resistivity and thermoelectric power measurements were performed over the temperature range of 77–300K and 296–373K, respectively. Hall effect and optical transmission measurements were preformed at room temperature. All these studies showed that the structural, electrical as well as the optical characteristics of Zn1−xGaxO films depend very sensitively on the Ga contents. As the Ga doping concentration is increased, initially an increase in carrier concentration and optical band gap is observed (until x=0.04), which is followed by a decrease at higher concentrations. These features were attributed to the combined effect of band filling (Burstein-Moss effect), electronic correlation, and epitaxial strain present in the system. Above parameters also affected the electrical properties of the films quite significantly. Zn1−xGaxO films with 1% of Ga doping (x=0.01) showed metal-like electrical resistivity. However, for higher doping levels, enhanced scattering potential, arising from randomly distributed impurity atoms, resulted in the Anderson localization of electronic states.

Preparation of Epitaxial YBCO Films by a Novel Excimer-Laser-Assisted MOD

Epitaxial (YBCO) films were prepared on cerium oxide-buffered (40 nm)-cut single-crystal sapphire substrates by a novel excimer-laser-assisted metalorganic deposition (ELAMOD). Laser irradiation after application of a fluorine-free ingredient solution onto the substrates revealed to have the advantages of shorter heating (prefiring and heat-treatment) time and higher superconducting current density . The in excess of 5 was measured by an inductive method at 77.3 K. The laser irradiation is considered to produce a desirable precursor, in which three metal components are well-mixed in short time. The X-ray diffraction measurement of the irradiated area clearly showed a complete -axis oriented YBCO and no impurity phase, whereas -axis oriented YBCO was also observed in the area without irradiation. The ratio of the -axis component decreased with an increase of the laser irradiation.

PLD-grown Yb-doped Sesquioxide Films on Sapphire and Quartz Substrates

This paper focuses on the fabrication and characterization of crystalline Yb-doped sesquioxide films (Yb2O3, Yb:Lu2O3, Yb:Sc2O3) grown by pulsed laser deposition on single crystal sapphire (α-Al2O3) and quartz (α-SiO2) substrates as well as on lutetia (Lu2O3) and scandia (Sc2O3) substrates. Such ytterbium doped active films can be promising for use in a thin-disk laser setup. X-Ray diffraction measurements show that the films grow highly textured along <111> direction. The ω-scan (rocking curve) shows deviation of the crystallite orientation ∼4° in case of Lu2O3 and Yb2O3 films and <1° for Sc2O3 films. Yb2O3 films reveal no luminescence at room temperature and the well known Yb3+ emission in the 975-980 nm region (zero-phonon line) can only be observed at temperatures below 20 K. A similar effect is observed in a bulk Yb2O3 crystal. Ytterbium emission and excitation spectra measured at room temperature for Yb(5%):Lu2O3 and Yb(4%):Sc2O3 films resemble those of the bulk crystal very closely. Luminescence decay lifetimes are also comparable to those measured in a bulk crystal. This indicates a high quantum efficiency of the Yb3+-emission and allows application of such films as active media for thin disk lasers.

Epitaxial growth and magnetic properties of La0.7Sr0.3MnO3 films on (0001) sapphire

In this letter the authors report the growth of (110) oriented La0.7Sr0.3MnO3 films on (0001) single-crystal sapphire substrate. Growth of La0.7Sr0.3MnO3 (LSMO) films with a fixed orientation was achieved by carefully controlling the deposition conditions, specifically the substrate temperature. The fixed orientation relationship between the film and the sapphire substrate ((110)f‖(0002)s, (1¯10)f‖(01¯10)s, (002)f‖(2¯110)s), has been explained via domain matching epitaxy, where there is perfect matching in one direction and 19% planar misfit in the perpendicular direction. LSMO films showed interesting magnetic anisotropy along the two mutually perpendicular in-plane directions, corresponding to [002]f as strong and [1¯10]f as weak magnetization directions.

Evolution with temperature of a single sol–gel TIO 2 layer on sapphire (α-Al 2O 3) (0 0 0 1)

TiO 2 sol–gel layer has been deposited on single crystal sapphire (0 0 0 1) substrate. Evolution of the layer microstructure with the thermal treatment in the range 100–1100 °C has been studied using X-ray diffraction and X-ray reflectometry. TiO 2 layer density first increases with temperature up to 800 °C and then decreases with the appearance of a high roughness finally leading to anatase islands formation. The single crystal nature of the substrate seems to contribute to hinder the transformation of the anatase phase into the rutile phase and to induce a preferred orientation of the TiO 2 islands.

Photoinduced insulator-to-metal phase transition inVO2crystalline films and model of dielectric susceptibility

A light-induced insulator-to-metal ultrafast phase transition (PT) was observed in $\mathrm{V}{\mathrm{O}}_{2}$ thin films deposited on single-crystal sapphire and amorphous glass substrates. The PT and optical properties of $\mathrm{V}{\mathrm{O}}_{2}$ were characterized by transient reflection, absorption, and transient grating nonlinear optical measurements. $\mathrm{V}{\mathrm{O}}_{2}$ films deposited on crystalline substrates are spatially ordered and show noticeable anisotropy in optical response upon PT. Structural and optical properties are dependent on the shear plane of the sapphire substrate. Significant twinning was found for $\mathrm{V}{\mathrm{O}}_{2}$ deposited on a $(001){\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ substrate while a $\mathrm{V}{\mathrm{O}}_{2}$ film on $(012){\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ is highly textured, without detectable twinning. $\mathrm{V}{\mathrm{O}}_{2}$ dielectric susceptibility is described by a proposed model. The V-O-V charge transfer was considered as a major process contributing to the linear and nonlinear dielectric susceptibility. The PT time and relaxation dynamics were found to be dependent on the film morphology and concentration of structural defects.

Comparison of non-polar ZnO [formula omitted] films deposited on single crystal ZnO [formula omitted] and sapphire [formula omitted] substrates

Homo- and heteroepitaxial growth of non-polar Zinc oxide (ZnO) ( 1 1 2 ¯ 0 ) films was achieved on single crystal ZnO ( 1 1 2 ¯ 0 ) and sapphire ( 0 1 1 ¯ 2 ) substrates by an atmospheric pressure metal-organic chemical vapor deposition (MOCVD) using zinc acetylacetonate and oxygen gas. The full-widths at half-maximum (FWHM) of X-ray diffraction patterns of grazing incident diffraction measurement of homoepitaxial ZnO films were smaller by one order of magnitude than those of heteroepitaxial ZnO films. A striped pattern along the c-axis of ZnO was clearly observed in homoepitaxial films deposited at 650 °C. Room temperature photoluminescence spectra of homoepitaxial films exhibited only a strong near-band-edge emission. Free exciton emissions were clearly observed in the photoluminescence spectra of homoepitaxial ZnO films at 5 K.

Supercurrent density above 106A∕cm2 at 77K in a single-crystal film conductor of the cuprate high-Tc superconductor YBa2Cu3O7−δ—dream or reality?

Supercurrent transport phenomena in c-oriented epitaxial thin films of the the high-Tc superconducting (HTS) cuprate YBa2Cu3O7−δ (YBCO) with a high critical current density Jc(77K)⩾2×106A∕cm2 are investigated by four-probe transport measurements, low-frequency magnetic susceptibility studies, and SQUID magnetometry. The film samples are deposited on a single-crystal sapphire (r-cut) substrates with a CeO2 or LaAlO3(100) buffer layer by off-axis dc magnetron sputtering or pulsed laser ablation. A model of the mechanisms of Abrikosov vortex pinning and supercurrent limitation is developed and discussed by comparing its predictions with the results of measurements of the critical current and its dependence on applied magnetic fields of different strength and orientation and also with nanostructure data obtained by high-resolution transmission electron microscopy and electron diffraction in a backscattering geometry. It is shown that the low-angle subboundaries (LABs) formed between domains with a slight azimuthal misorientation during the epitaxial growth of the film play a key role in the phenomena observed in the transport of supercurrent. The tilt LABs form equidistant ordered rows of edge dislocations with nonsuperconducting cores about 3–4nm in diameter. The dislocation lines in the LABs are parallel to each other and perpendicular to the film plane. The average density of dislocations over the area of the film depends on the real statistics of the random system of LABs and can reach 1011cm−2. Since the diameter of the “normal” core of a dislocation is close to the diameter of the core of an Abrikosov vortex, the elementary pinning force of the vortex to the core of the dislocation is close to the maximum possible. The pinning on dislocation subboundaries has the following characteristics: 1) the achievement of high values Jc(77K)⩾2×106A∕cm2 and Jc(20K)&amp;gt;107A∕cm2 in epitaxial films and conductors; 2) the existence of a “plateau” on the Jc(H) curve, i.e., Jc(H)=const for H&amp;lt;Hm; 3) a logarithmic decline of Jc(H) for H&amp;gt;Hm, i.e., at the transition from the single-particle pinning regime to the collective pinning of the vortex lattice on the statistical ensemble of randomly distributed dislocation subboundaries; 4) the existence of a threshold field Hp that determines the limit up to which the vortices in a thin film (d⩽λ) remain rectilinear and perpendicular to the film even in a field inclined at a large angle; 5) the evolution of the angle dependence of Jc(θ) with a change of field strength is in complete agreement with the model of dominant pinning on “threading” edge dislocations. A new “peak effect”—an increase of Jc(H‖ab) with increasing longitudinal field—is observed for the first time for H&amp;gt;Hm, i.e, after the end of the “plateau” Jc(H‖ab)=const.

Ferromagnetism in epitaxial germanium and silicon layers supersaturated with managanese and iron impurities

The possibility of laser synthesis of diluted magnetic semiconductors based on germanium and silicon doped with manganese or iron up to 10–15 at % has been shown. According to data on the electronic levels of 3d atoms in semiconductors, Mn and Fe impurities are most preferable for realizing ferromagnetism in Ge and Si through the Ruderman-Kittel-Kasuya-Yosida mechanism. Epitaxial Ge and Si layers 50–110 nm in thickness were grown on gallium arsenide or sapphire single crystal substrates heated to 200–480°C. The content of a 3d impurity has been measured by x-ray spectroscopy. The ferromagnetism of layers and high magnetic and acceptor activities of Mn in Ge, as well as of Mn and Fe in Si, are manifested in the observation of the Kerr effect, anomalous Hall effect, high hole conductivity, and anisotropic ferromagnetic resonance at 77–500 K. According to the ferromagnetic resonance data, the Curie point of Ge:Mn and Si:Mn on a GaAs substrate and of Si:Fe on an Al2O3 substrate is no lower than 420, 500, and 77 K, respectively.

Structure of (Ba0.7Sr0.3)TiO3 films prepared by chemical solution deposition on sapphire substrates

Barium strontium titanate (BST) films on single-crystal sapphire substrates are prepared by chemical solution deposition upon annealing at temperatures T = 700, 850, and 1000°C. The structure of the BST films is investigated using transmission electron microscopy, high-resolution electron microscopy, and x-ray diffraction. It is established that, upon annealing at T = 700 and 850°C, the film crystallizes in the tetragonal phase of the (Ba0.7Sr0.3)TiO3 perovskite without texture and transition layers. The mean grain sizes are 17 and 37 nm, respectively. However, an increase in the annealing temperature to 1000°C brings about a decrease in the mean grain size to 25 nm and the appearance of additional phases due to the interaction at the film-substrate interface.